Why must exposure time increase by a factor of four when the source-to-film distance is doubled?

The correct answer is based on the principle known as the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source. As the distance from the source to the film increases, the radiation spreads out over a larger area, leading to a decrease in intensity.

When the source-to-film distance is doubled, the radiation intensity decreases to one-fourth of what it was at the original distance. This means that to maintain the same exposure on the film, the exposure time must be increased by a factor of four to compensate for this reduction in intensity. By increasing the exposure time, you ensure that the film receives enough radiation to achieve the desired density and image quality.

This understanding of the relationship between exposure time and distance is crucial in radiographic testing, as it directly affects the quality and readability of the radiographic images produced.